The present invention relates to a compression-molding material and a method for producing a softly magnetic composite, using such a compression-molding material.
Magnetically soft composites are needed for producing temperature-, corrosion-, and solvent-resistant, magnetic component parts, particularly in electromechanics. In this context, these magnetically soft composites, and the component parts produced using them, need to have certain properties: They should have a high magnetic permeability, a high magnetic saturation, a low coercive field strength, and a specific electrical resistance that is as high as possible. The combination of the mentioned magnetic properties with a specific electrical resistance produces high switching dynamics with low eddy current losses, meaning that such a component part is magnetically saturated and demagnetized within a short time.
A magnetically soft, malleable composite and a method for producing it were already suggested in German Patent Application No. 197 35 271 A1, wherein a powder having magnetically soft properties is coated with a thermoplastic compound, and subsequently is pressed into a molded article. The molded article or the formed compression-molding material is then subjected to heat treatment, which is in an atmosphere of inert gas, and exceeds the melting point of the thermoplastic compound.
In addition, it is already known that non-alloyed or alloyed iron powder can be axially compression-molded with thermoplastic resins, such as epoxides or phenolic resins.
In comparison with the related art, the compression-molding material of the present invention and the method of the present invention for producing a magnetically soft composite, using such a compression-molding material, have the advantage that the temperature that was required until now while molding the compression-molding material in a compression-molding die, for example, in a bottom die, can be lowered. At the same time, one can dispense with preheating the compression-molding material prior to molding it. The improved sliding behavior of the compression-molded material also allows the portion of thermoplastic compound in the compression-molded material to be reduced.
Furthermore, the compression-molding material of the present invention allows higher material densities to be attained at a given compression force, and reduces the tool wear. Dispensing with the preheating of the compression-molding material prior to molding prevents undesired oxidation, e.g. of iron powder as a starting powder having magnetically soft properties.
In the method according to the present invention, the lowering of the mold temperature also illuminates the need to mold in the compression-molding die, in the presence of inert gas.
Finally, the compression-molding material according to the present invention and the method according to the present invention have the advantage of the processing being easier due to the considerably simplified hot-pressing device, as well as the lower energy consumption during the molding.
Thus, the magnetically soft composite, or component parts, are advantageously produced using this composite, by uniaxially pressing the compression-molding material in a female mold, at temperatures lower than the melting temperature of the thermoplastic compound added to the compression-molding material; and by subsequently subjecting the compression molding material to a stepped, thermal aging process.
In this aging process, the added lubricant is initially evaporated or pyrolyzed at temperatures below the melting temperature of the thermoplastic compound, and subsequently, the thermoplastic compound is melted by increasing the temperature further. In this connection, the melted, thermoplastic compound wets the powder particles of the raw powder having the magnetically soft properties, and therefore, effectively bonds the powder particles after cooling. This results in the attained composite having a high mechanical strength and a high electrical resistance.
The compression-molding material of the present invention, which is used as a starting material for the method of the present invention for producing the magnetically soft composite, starts out from a magnetically soft powder that is either coated on the surface by a thermoplastic compound, or is alternatively dry-mixed with a fine thermoplastic powder. To that end, the powder particles can be coated by the thermoplastic compound, e.g. by adding a solution of a suitable thermoplastic polymer in a solvent.
In the case of dry-mixing the thermoplastic compound with the magnetically soft powder, a powdery, thermoplastic compound is used which preferably has an average particle size of 1 xcexcm to 100 xcexcm, and especially 5 xcexcm to 40 xcexcm.
As a means of lubrication, a lubricant is used which evaporates, or else thermally decomposes and volatizes, in response to the compression-molding material being heated in an atmosphere of inert gas, during the two-stage, thermal aging process, at temperatures below the melting point of the applied thermoplastic compound. Neither the lubricant nor its decomposition products react chemically with the thermoplastic compound and/or the raw powder having the magnetically soft properties.
In order to prevent the molten thermoplastic from being expelled from the composite due to the pressure of the gases originating from the lubricant, it is also very advantageous, after molding at temperatures below the melting point of the thermoplastic compound, to first remove the lubricant, at least almost completely, from the compression-molding material before the thermoplastic compound is then melted by a further temperature increase, and the magnetically soft, raw powder is wetted.
So, on the whole, a considerable increase in the strength of the lubricant-free composite due to the formation of adhering polymer bridges occurs in response to the compression-molding material, or the component part produced from it, cooling off.
In addition, the lubricant is advantageously prevented from remaining in the structure of the attained, magnetically soft composite, and from negatively influencing its working properties there.
In view of the temperatures during molding and during the thermal aging process, the use of the lubricant stearic acid, which is also used simultaneously as a mold release agent, has proved to be advantageous. For this purpose, the stearic acid is advantageously added to the compression-molding material as a micronized powder having an average particle size of 1 xcexcm to 100 xcexcm, and especially 10 xcexcm to 50 xcexcm.
Several polymers are advantageously suitable as thermoplastic compounds, polyphenylene sulfide may be used. The combination of stearic acid with polyphenylene sulfide is particularly advantageous.